1. Field of the Invention
The present invention relates to a laser apparatus.
2. Description of the Related Art
In recent years, intensive study has been made in the medical field on an optical imaging technique for irradiating a living body with light outputted from a light source, such as a laser, and imaging signals generated by interaction between the incident light and a tissue of the living body. The photoacoustic tomography (PAT) is one of such optical imaging techniques. The PAT includes irradiating a living body with pulsed light outputted from a light source and then detecting an acoustic wave generated from a tissue of the living body that has absorbed the pulsed light. The acoustic wave generated by such a photoacoustic effect is also called a “photoacoustic wave”.
A segment to be examined, such as a tumor, often exhibits a high absorptivity and swells instantaneously upon irradiation by absorbing a larger amount of optical energy than peripheral tissues. By detecting a photoacoustic wave generated upon the swelling by using an acoustic wave detector and then analyzing the signal of the photoacoustic wave, a sound pressure distribution of the photoacoustic wave generated by the photoacoustic effect in the tissue of the living body can be imaged. Hereinafter, the image thus obtained will be referred to as a “photoacoustic image”.
The photoacoustic image can be converted to an optical characteristic distribution in a living body, particularly to an absorption coefficient distribution. Such information can be utilized in quantitatively measuring a substance in a subject to be examined, for example, glucose or hemoglobin contained in blood. In recent years, intensive research on a photoacoustic imaging apparatus has been proceeding for the purpose of applying a blood vessel image obtained by the PAT to imaging and diagnosis of a breast cancer or the like.
A substance in a living body, such as glucose or hemoglobin, differs in absorptivity depending on the wavelength of incident light. Therefore, the distribution of the substance in the living body can be measured more precisely by irradiating the living body with different wavelengths of light and analyzing the difference between resulting absorption coefficient distributions. Usually, light having a wavelength ranging from 500 nm to 1,200 nm is used as irradiation light. Particularly when absorption of melanin or water has to be avoided, near-infrared light ranging from 700 nm to 900 nm is used as irradiation light.
As a light source capable of outputting a plurality of wavelengths of light within the wavelength range noted above, a tunable laser apparatus is known which uses a titanium sapphire crystal or an alexandrite crystal as a gain material. Such a tunable laser apparatus has an optical resonator (cavity) in which the titanium sapphire crystal or the alexandrite crystal is placed and is capable of outputting different wavelengths of light by switching an oscillation wavelength of the cavity to another.
In “Flashtube-pumped Dye laser with Multiple-Prism Tuning,” Applied Optics/Vol. 10, No. 6, P. 1348/June (1971), (NPL 1: Non Patent Literature 1), there is described a laser apparatus configured to output different wavelengths of light by changing the angle formed between a prism and a mirror which form a cavity. FIG. 7 is a schematic view illustrating a wavelength switching mechanism of the tunable laser apparatus described in “Flashtube-pumped Dye laser with Multiple-Prism Tuning,” Applied Optics/Vol. 10, No. 6, P. 1348/June (1971).
In FIG. 7, light having passed through a prism 7001 is refracted at an angle in accordance with the wavelength thereof. That is, different wavelengths of light having passed through the prism 7001 are refracted at different angles by wavelength dispersion (dθ/dλ) of the refraction angle of the prism.
Only that light beam 7007 of light beams refracted at different angles which is perpendicularly incident on a mirror 7004 is reflected by the mirror 7004 and returned into the cavity. The light beam 7007 thus returned into the cavity reciprocates within the cavity and is given a gain by a gain member 7002 to generate oscillation.
In FIG. 7, the mirror 7004 is mounted on a mirror rotating mechanism 7006. The mirror rotating mechanism 7006 rotates the mirror 7004 while positioning the mirror 7004 perpendicularly to a plane including incident and reflected light having been subjected to wavelength dispersion (i.e., a plane parallel to the drawing sheet of FIG. 7). With the mirror 7004 thus rotating, light beams of different wavelengths become perpendicularly incident on the mirror 7004 in accordance with the angle of rotation of the mirror 7004 and are returned into the cavity. In this way, different wavelengths of light can be oscillated by changing the angle of rotation of the mirror 7004.
A technique described in Japanese Patent Application Laid-open No. 2005-123330 (PTL 1: Patent Literature 1) is also known.
Non Patent Literature 1: “Flashtube-pumped Dye laser with Multiple-Prism Tuning”, Applied Optics/Vol. 10, No. 6, P. 1348/June (1971)
Patent Literature 1: Japanese Patent Application Laid-open No. 2005-123330